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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import Control.Monad (void, when)
+import Data.Functor
+import qualified Data.Heap as H
+import qualified Data.List as L
+import qualified Data.Map as M
+import qualified Data.Vector as V
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+
+exampleExpectedOutput = 11048
+
+data Tile = Wall | Floor | Start | End deriving (Eq, Show)
+type Line = V.Vector Tile
+type Input = V.Vector Line
+
+type Parser = Parsec Void String
+
+parseTile :: Parser Tile
+parseTile = char '#' $> Wall
+ <|> char '.' $> Floor
+ <|> char 'E' $> End
+ <|> char 'S' $> Start
+
+parseLine :: Parser Line
+parseLine = do
+ line <- some parseTile <* eol
+ return $ V.generate (length line) (line !!)
+
+parseInput' :: Parser Input
+parseInput' = do
+ line <- some parseLine <* eof
+ return $ V.generate (length line) (line !!)
+
+parseInput :: String -> IO Input
+parseInput filename = do
+ input <- readFile filename
+ case runParser parseInput' filename input of
+ Left bundle -> error $ errorBundlePretty bundle
+ Right input' -> return input'
+
+type Cost = Int
+data Heading = N | S | E | W deriving (Eq, Show)
+type Coord = (Int, Int)
+data Position = Position Coord Heading Cost deriving Show
+instance Ord Position where
+ compare (Position _ _ c1) (Position _ _ c2) = c1 `compare` c2
+instance Eq Position where
+ (Position _ _ c1) == (Position _ _ c2) = c1 == c2
+type Visited = M.Map Coord Int
+
+type Candidates = H.MinHeap Position
+
+compute :: Input -> Int
+compute input = walk M.empty $ H.singleton (Position start E 0)
+ where
+ walk :: Visited -> Candidates -> Int
+ walk v h | t == End = c
+ | otherwise = walk v' $ H.union h' $ H.fromList $ nexts v' $ Position p d c
+ where
+ ([(Position p@(x, y) d c)], h') = H.splitAt 1 h
+ t = input V.! y V.! x
+ v' = case M.lookup p v of
+ Just c' -> if c < c' then M.insert p c v else v
+ Nothing -> M.insert p c v
+ nexts :: Visited -> Position -> [Position]
+ nexts v p = L.filter (valid v) $ candidates p
+ valid :: Visited -> Position -> Bool
+ valid v (Position p@(x, y) _ c) = input V.! y V.! x /= Wall && case M.lookup p v of
+ Just c' -> c < c'
+ Nothing -> True
+ candidates :: Position -> [Position]
+ candidates (Position (x, y) N c) = [ Position (x-1, y) W (c+1001), Position (x+1, y) E (c+1001), Position (x, y-1) N (c+1) ]
+ candidates (Position (x, y) S c) = [ Position (x-1, y) W (c+1001), Position (x+1, y) E (c+1001), Position (x, y+1) S (c+1) ]
+ candidates (Position (x, y) E c) = [ Position (x, y-1) N (c+1001), Position (x, y+1) S (c+1001), Position (x+1, y) E (c+1) ]
+ candidates (Position (x, y) W c) = [ Position (x, y-1) N (c+1001), Position (x, y+1) S (c+1001), Position (x-1, y) W (c+1) ]
+ height = V.length input
+ width = V.length (input V.! 0)
+ start = (1, height - 2)
+
+main :: IO ()
+main = do
+ example <- parseInput "example"
+ let exampleOutput = compute example
+ when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+ input <- parseInput "input"
+ print $ compute input